Simultaneous transmission of multiple data streams is carried out in a MIMO communication system that employs multiple (NT) transmission antennas and multiple (NR) receiving antennas. Depending on the usage, MIMO system contributes to improvement of performance by spatial diversity or contributes to increase of system capacity by spatial multiplexing. The presence of random fading and multipath delay spread in a wireless communication system enables such improvements.
The multiple communication channels present between the transmission antennas and receiving antennas usually change with time and have different link conditions. MIMO systems having feedback provide the transmitter with the channel state information (CSI), allowing the use of methods such as link adaptation and water filling to provide a higher level of performance.
A well-known technique to increase data rate by spatial multiplexing is discussed in Non-Patent Document 1.
Spatial diversity is implemented by space-time block coding, which provides the full advantage of diversity. The space-time block code is disclosed, for example, in Non-Patent Document 2.
MIMO techniques were first designed assuming a narrowband wireless system, namely a flat fading channel. Therefore, it is difficult to achieve high effects in frequency selective channels. OFDM is used in conjunction with MIMO systems to overcome the frequency selective channels proposed by the wireless environment.
OFDM is capable of converting the frequency selective channel into a set of independent parallel frequency-flat subchannels using the inverse fast Fourier transform (IFFT). The frequencies of these subchannels are orthogonal and mutually overlapping, thereby improving spectral efficiency and minimizing inter-carrier interference. Attaching a cyclic prefix to the OFDM symbol further reduces the multipath effects.
With future technology shifting to accommodate a high speed service with increased IP dependency, it is necessary to meet requirements such as spectral efficiencies, system user capacity, end-to-end latency, and quality-of-service (QoS) management. While MIMO-OFDM systems meet some of these criteria, ARQ techniques also play an important role in ensuring fast and reliable delivery.
ARQ is a technique for transmitting a retransmission request for received packet data upon detecting an error in the received packet data. With the transfer of a large volume of high-speed data, more efficient ARQ techniques are typically used to reduce the number of retransmission requests.
It is obviously shown that Hybrid ARQ (HARQ) techniques include chase combining and incremental redundancy and improve efficiency by reducing ARQ overheads. HARQ techniques are primarily designed assuming a single-antenna transmitter and receiver.    Non-Patent Document 1: V-BLAST: an architecture for realizing very high data rates over the rich-scattering wireless channel” by P W Wolniansky et al in the published papers of the 1998 URSI International Symposium on Signals, Systems and Electronics, Pisa, Italy, Sep. 29 to Oct. 2, 1998.    Non-Patent Document 2: Tarokh, V., Jafarkhani, H., Calderbank, A. R.: Space-Time Block Codes from Orthogonal Designs, IEEE Transactions on information theory, Vol. 45, pp. 1456-1467, July 1999, and in WO 99/15871.